Legislation requires that emissions of nitrous oxides (NOx) from vehicle engines be reduced. Selective catalytic reduction (SCR) has been proposed to treat the exhaust gas stream of such engines, particularly diesel engines. In SCR a reductant, such as ammonia or urea, is mixed with the exhaust gases upstream of a catalytic chamber. When the exhaust gases are within a prescribed temperature range a chemical reaction occurs in the catalytic chamber to convert the reductant/NOx mixture into nitrogen and water. NOx emissions from the tailpipe are thus reduced. Several kinds of reductant are available, and one sub-set is collectively termed Diesel Exhaust Fluid (DEF).
This specification refers frequently to diesel engines of vehicles; however the invention is applicable to all internal combustion engines requiring NOx treatment of the exhaust gases in an SCR device. Such engines may also have a non-vehicle application.
A fluid reductant, such as urea, is generally supplied as a liquid to be dispensed to the exhaust system from a supply tank. Periodically, and typically at an interval of 15-20,000 miles in a vehicle, the tank must be replenished, for which purpose an indicator of low level may be provided.
In order to ensure that a fluid reductant of the prescribed quality is added to the supply tank, the efficiency of SCR conversion is monitored. Such monitoring is necessary not only to ensure continuing compliance with a legislative limit, but also to ensure that the engine performs as intended without generating indirect malfunction indications.
Vehicle legislation typically requires an on-board diagnostic (OBD) to test for correct operation of the SCR device, thereby to ensure that the emissions from the exhaust tailpipe remain within the legislative limit over time. Failure of the SCR device can thus be detected and signalled to the vehicle driver.
In a conventional vehicle OBD, the NOx content upstream of the SCR device is periodically compared with the NOx content downstream of the SCR device to ensure that conversion efficiency exceeds a predetermined minimum. The minimum conversion efficiency is selected according to requirements, and may for example lie in the range 10-40%; it is highly variable dependent upon the kind of engine and conditions of use, and the threshold will be determined by a skilled technician according to operational and design factors. Failure to meet this threshold typically illuminates a malfunction indicator light (MIL) on the vehicle dashboard, and logs a record in the usual electronic control unit (ECU) of the vehicle for later diagnosis by a repair technician.
Upstream NOx concentration may be calculated or may be sensed by a NOx sensor; downstream NOx is usually sensed by a separate NOx sensor.
A second OBD may be required to indicate the reason for failure of the SCR device. Two failure modes are generally possible, namely gross failure of the SCR device and incorrect fluid reductant. These failure modes cannot be distinguished by mere testing of NOx conversion efficiency.
In a prior art system NOx conversion efficiency is compared with a threshold for a test period immediately following a fluid reductant re-fill event. Such an event can be identified by a change in state of a level sensor in the supply tank. If NOx conversion efficiency is below the required threshold, the failure is determined to be low quality reductant rather than gross failure of the SCR. This result is logged electronically for a diagnostic technician, and may help to avoid misdiagnosis at a service centre. After the test period has passed, a failure to meet the threshold conversion efficiency may be assumed to be due to gross failure of the SCR, since the fluid reductant is considered to be of consistent quality until the next re-fill event.
It is apparent that the prior method can be defeated by regular topping-up of the fluid reductant if the amount of refill does not trip refill detection made by a level sensor. In such circumstances, topping up of the supply tank with e.g. water, will eventually cause NOx conversion efficiency to fall below the required threshold, but the failure will be logged as gross failure of the SCR rather than poor quality reductant, because no indication of re-fill has been detected. A particular circumstance where the existing method may fail is where the reductant supply tank has a separate header tank without level sensor, by reason of a confined space.
What is required is a means of differentiating a failure due to poor quality fluid reductant. Quality sensors have been proposed for fluid reductant, but are considered to be insufficiently accurate/too expensive at this time.